The present invention relates to a static seal structure for subdividing a space between a casing and a stationary sleeve received within the casing into a plurality of chambers. The static seal finds particular use between a casing and a sleeve utilized in a hydraulic control mechanism for gas turbine engines.
A known hydro-mechanical control for a gas turbine engine is illustrated in FIG. 1 and generally comprises a stationary casing 1, made of a lightweight alloy, within which a plunger 2 is movable along its longitudinal axis in the directions of arrows 2a. A stationary sleeve 3 is located within the casing 1 between the interior of the casing and the plunger 2. The sleeve 3 is usually made of a very hard steel to provide a long lasting bearing surface for the plunger 2 and to subdivide the space between the sleeve and the casing into a plurality of chambers. Means are provided between the sleeve and the casing to effect a fluid-tight seal between these elements so as to subdivide the space into the chambers for interconnection, in known fashion, to a hydraulic power circuit.
The casing 1 is generally cylindrical and defines a plurality of chambers 1a, 1b and 1c, each having a diameter D.sub.1 (radius R.sub.1) separated by sealing bosses 1d and 1e. Each of the sealing bosses define a sealing surface having a diameter d.sub.1 (radius r.sub.1) which is less than the diameter D.sub.1 (r.sub.1 less than R.sub.1).
The external surface of the generally cylindrical sleeve 3 is also divided into chambers 3a, 3b, and 3c separated by sealing bosses 3d and 3e, respectively. O-rings 4 are located in grooves 5 formed in each of the sealing bosses 3d and 3e, respectively. The O-rings contact the sealing bosses 1d and 1e to provide a fluid tight seal between the respective chambers. The edges of the sealing bosses 1d and 1e may be bevelled to facilitate the insertion of the sleeve 3 with the O-rings 4 in order to prevent the tearing or marring of the sealing surfaces of the O-rings.
While the known structure has generally proven to be satisfactory in performance, fabrication defects are frequently observed in, particularly, the diameter d.sub.1 of the sealing bosses. If, during fabrication, these sealing surfaces are machined to an excessive degree, diameter d.sub.1 will be too large, allowing clearance between the sealing surface and the O-rings 4. This results in an imperfect fluid seal between these elements thereby rendering the device inoperative. Due to the difficulties in re-shaping such an excessively machined sealing boss, the casings thus formed were deemed to be defective and the entire casing had to be discarded.